The ability to monitor the chemical composition and physical parameters of gaseous environments, including indoor air quality, atmospheric measurements for weather forecasting, worker safety in hazardous environments and closed spaces, processing gases, and vehicle and plant exhaust, has been an important goal for several reasons. For example, the detrimental environmental effects of toxic species such as formaldehyde, carbon monoxide, ozone, hydrocarbons, chlorocarbons, nitrogen oxides and aromatics have led to the need to develop efficient, sensitive, and affordable ways of detecting the composition and presence of such toxic substances. Additionally, the efficiency of chemical processes, in terms of energy and raw material used per unit product or service delivered, relies on the ability to reliably sense deviations from the optimal processing conditions. Development of measurement and control technologies, including advanced sensor technology, is a critical component for lowering raw materials and energy consumption, improving the productivity and reducing generation of waste and pollutants. On the other hand, a primary concern for health care facilities as well as for other work place environments, are volatile organic compounds, such as alcohols, chlorinated hydrocarbons, aromatic solvents, and formaldehyde. Furthermore, humidity, temperature, and pressure are important environmental factors, which are of importance to both human comfort and to many industries and technologies, such as the production of electronic devices, precision instruments, food products, agriculture, horticulture, green houses and meteorology. Low-cost, light-weight sensors for routine balloon-borne measurements of water vapor concentration are required to provide one percent accuracy with high time resolution measurements at altitudes up to 20 km, low vapor concentration, and at temperatures down to at least minus 60° C.
Although physical, chemical, and biochemical sensing have been extensively researched, real-time monitoring and control leaves much to be desired. The following sensor performance parameters are in need of improvement for many applications: reliability and robustness, miniaturability, real time response, sensitivity and selectivity, built-in self-calibration, survivability in harsh application environments, low cost and manufacturability. Existing sensors have several inherent limitations that limit their performance, the most restraining are: cost, lack of selectivity, low reliability and durability, slow response, false response, the need for significant power, resistance to poisoning by sulfur-containing compounds and instability (hysteresis).
Existing approaches for high temperature microsensors are based mainly on planar technologies: the functional components (sensitive element, electrodes, temperature sensor, heating structures, and passivation) are made using standard thin or thick film deposition. Although significant breakthroughs were achieved in the development and fabrication of thin film microsensors, their performance parameters, especially sensitivity, reliability and working temperature range, still limits their usability. Moreover, portable sensor arrays capable of detecting multiple toxic air pollutants, such as volatile organics, carbon monoxide, formaldehyde, nitrogen oxides, and other air components, such as carbon dioxide and humidity are not available.
Recent advances in nanotechnology can greatly improve the sensor performance due to the possibility of tailoring the microstructure and chemistry at the nanoscale level, thus enhancing the gas-solid interaction. Although significant progress was achieved in different types of microsensors, synthesis of nanostructured high surface area sensing elements on intrinsically flat non-porous micromachined substrates is still a problem. Thin film microsensors fabricated on diaphragms, which are thermally decoupled from the substrate, demonstrated significant advantages in response time, and potential for array integration in comparison with larger (thick film) sensors. Nevertheless, there are several outstanding problems with existing thin film sensors. Small amounts and low surface area of sensitive material leads to lower signal-to-noise ratio (sensitivity), and easy loss of sensitivity on exposure to sulfur-containing compounds, as well as poor overall durability.
Nanostructured materials, with their small grain size, large number of grain boundaries and high specific area present new opportunities for the development and the commercialization of the next generation of gas sensors for air quality monitoring and control with significantly improved properties. The main challenge in realizing such opportunities is tailoring the sensing materials morphology and the composition at the nanometer scale. Furthermore, it is important that the nanostructured sensing element is manufactured in a form that is amenable to sensing device architectures and with ppb reproducibility.
If these issues were resolved, significant performance advantages could be realized. Low thermal mass and fast response microsensors could allow real time process monitoring and control, as well as will enable implementation of programmable temperature/voltage modulation for higher sensitivity and stability, elimination of drift and noise, and self-calibration. This invention addresses this opportunity.
Prior publications related to producing gas sensors include those of U.S. Pat. No. 4,631,952 which teaches a method of preparing a sensor by the formation of a dispersion of conducting particles with a material capable of swelling in the presence of the liquid, gas or vapor to be sensed. U.S. Pat. No. 4,984,446 teaches the preparation of a gas detecting device by a layer by layer build up process, and U.S. Pat. No. 4,988,539 teaches an evaporation-deposition method and process for manufacturing a gas detection sensor. U.S. Pat. No. 5,387,462 teaches a method of making a sensor for gas, vapor, and liquid from a composite article with an electrically conductive surface having an array of whisker-like microstructures with an aspect ratio of 3:1 to 100:1. U.S. Pat. No. 5,345,213 teaches a method for fabrication of temperature-controlled micromachined arrays for chemical sensor fabrication and operation. Furthermore, U.S. Pat. No. 6,079,873 describes a microhotplate-based differential calorimeter for detecting gases and chemical reactions. Although these prior methods provide improved methods for producing sensors, there is still a need to develop sensors that are selective, sensitive to trace species, fast, small, accurate, reproducible, stable in extreme environments, durable, and finally affordable.
Prior publications relevant to nanoporous anodic alumina include U.S. Pat. Nos. 4,472,533; 5,174,883; 5,198,112; 5,202,290 and 5,581,091, which describe methods of depositing various materials inside the nanoscale pores of anodic alumina. Depending on the material deposited in the pores and the pore size, these materials were described as suitable for use as catalysts, microelectrodes, reverse osmosis membranes, or nanoelectric devices. However, none of these references describe the manufacture of nanotubes of sensing materials, nor do they suggest that gas sensing devices can be constructed by these methods.